This invention is in the technical field of extreme ultraviolet (EUV) lithography.
A number of factors can degrade optical performance of EUV lithography tools. Manufacturing errors in the projection optics mirrors, as well as thermally induced deformations in their illuminated regions during operation, can produce optical aberrations which will degrade image quality at the wafer. Image placement errors (distortion) can also occur. Since the reticle illumination is non-telecentric, changes in reticle height (caused for example by non-flatness of the reticle) can also produce distortion at the wafer.
It is therefore an object of this invention to provide a system for correcting aberration and distortion in EUV lithography, or a lithographic projection apparatus.
In EUV lithography, the reticle is reflective and is chucked in the lithography tool by its unpatterned surface, not around its periphery as required for a reticle of a transmissive type. Because the reticle is reflective, it is illuminated away from normal incidence. Consequently, any variation in height of the reticle-patterned surface will lead to displacement of features, or distortion, at the wafer. If either the back side of the reticle or the mating chuck surface is not flat, or if particles are trapped therebetween during chucking, furthermore, the front surface of the reticle will become distorted, leading to further feature displacement at the wafer.
The reticle-patterned surface is composed of a multi-layer film coating, and the reflectivity of the coating is unlikely to be much greater than 0.65. Thus, the reticle will absorb a significant amount of EUV power, and therefore must be actively cooled. Also, because the coating has a tensile stress, the reticle may be slightly bowed and must be flattened by the chuck. Finally, high throughput EUV lithography tools are calling for reticle stage accelerations of up to 8 g (g being the gravitational acceleration). The reticle must therefore be held very firmly to avoid slippage and possible deformation caused by the acceleration.
It is another object of this invention to provide a reticle chuck suitable for correcting aberration and distortion in EUV lithography, or a lithographic projection apparatus.